Modern motor vehicles often have an underpressure brake booster with at least two chambers which are separated by a diaphragm, which underpressure brake booster superimposes an auxiliary force, built up by means of a pressure difference between the chambers, on the activation force applied to the brake pedal by the driver. The resulting overall force acts on a master brake cylinder which brings about a buildup of pressure and therefore a braking force in the wheel brakes of the vehicle. In this context, there is a boosting factor of for example 4:1 between the overall force and the activation force, with the result that even heavy vehicles can be braked with moderate activation forces. Since the underpressure or the pressure difference which serves as an energy source decreases with each activation, the underpressure brake booster is usually evacuated using the intake section of an internal combustion engine. In modern internal combustion engines with direct injection and/or turbocharging, a sufficient underpressure in the intake section is not ensured in every operating state, in particular not in a certain time period after a cold start of the internal combustion engine.
EP 0 754 607 B1, which is incorporated by reference, discloses a brake installation for motor vehicles having a brake booster which generates an output braking force by superimposing an auxiliary force on the activation force, and having a hydraulic wheel braking force control system, by means of which a higher pressure than the pressure applied by the output braking force can be generated in the wheel brake cylinders. In the brake installation, an evaluation device, which detects the undershooting of a predefined ratio of the auxiliary force to the activation force, and a control device are provided, said control device generating a setpoint pressure in the wheel brake cylinders by means of the hydraulic wheel brake control system after this ratio has been undershot, which setpoint pressure at least partially compensates the effects on the actual pressure by the auxiliary force which decreases in relation to the activation force.
If the auxiliary force is no longer in a predefined ratio with respect to the activation force and an increase in the brake pressure then only occurs as a result of the activation force applied by the driver, the modulation point of the brake booster is reached. The modulation point is dependent on the structural configuration of the brake system and the available underpressure. Even in the event of a defect in the underpressure brake booster, it is desirable to assist the driver by means of an additional buildup of hydraulic pressure, for example by means of an electric hydraulic pump.
WO 03/068574 A1, which is incorporated by reference, discloses a method for applying a predefined variable brake pressure in the wheel brakes of a brake installation, in which, in a control and/or data processing system, input variables which determine the brake pressure in the individual wheel brakes are evaluated and manipulated variables of hydraulic valves are defined. A characteristic curve is stored in the control or data processing system, said characteristic curve correlating the valve current of the hydraulic valve with a pressure difference of the hydraulic valve, and in accordance with the characteristic curve an electric valve current, by means of which the hydraulic valve is controlled in an analogous fashion, is defined. The electric valve current is expediently determined on the basis of the pressure difference between the setpoint pressure for the wheel brakes and the pressure of an activation device. The pressure of the activation device (the tandem master brake cylinder) is determined by means of a pressure sensor arranged in a brake circuit.